feat: show pending resource replacement in the toolbar

[Maxxich]

Depends on PR: feat: connect resource replacement to the editor lifecycle.

Show a single toolbar spinner while resource replacement requests are pending,
including custom toolbar configurations. Add a demo and document the indicator.
Cover the default, custom and mobile toolbar while requests complete or are cancelled.
Register the resource guide in the generated documentation, repair its toolbar
backlink and apply the required demo formatting. The library implementation matches
the fixed reference; the approved documentation/formatting changes and additional
tests are documented in REPORT.md.

Summary by Sourcery

Display pending resource replacement activity in the editor toolbar and provide documentation, demonstrations, and coverage for the behavior.

New Features:

• Show a single pending resource replacement spinner in the main toolbar across editor modes, custom toolbar configurations, overflow states, and mobile layouts.
• Add an interactive resource replacement demo covering image and file resources, completion, cancellation, and concurrent operations.

Bug Fixes:

• Keep the resource replacement indicator visible until all pending operations have completed or been cancelled.
• Stabilize link visual test cursor placement.

Enhancements:

• Extend the flexible toolbar with support for reserving space for status content alongside toolbar items.

Documentation:

• Document the resource replacement toolbar indicator and link the resource replacement guide from toolbar customization documentation.
• Register the resource replacement guide in the generated documentation hierarchy and repair its toolbar backlink.

Tests:

• Add visual coverage for default, custom, and mobile toolbars in both WYSIWYG and markup modes, including concurrent requests, cancellation, and completion.

Chores:

• Apply required resource replacement demo formatting updates.

[gravity-ui]

[gravity-ui]

[sourcery-ai]

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